An example of such a filtering network is a basic low-pass filter of pseudo-elliptical type, having 11 poles and five transmission zeros. Such a network is able to allow the DVB-T signal situated in the band [462-862] MHz to pass and to reject the GSM transmission band found in the band [890-915] MHz.
A filtering network of this type is described in FIG. 1. The network is symmetrical. Consequently, the symmetrical rank elements have the same values. The network comprises:                coupling inductances L1 to L6 of values Ls1, Ls2, Ls3, connected in series between an input port P1 and an output port P2.        L/C series resonant elements of values Lr1/Cr1, Lr2/Cr2, Lr3/Cr3 connected in parallel between the connection points A1 to A5 of coupling inductances L1 to L6 and the ground.        
Between the connection point A1 of inductances L1 and L2 and the ground is thus connected a first resonant element Lr1/Cr1. Between the point A2 and the ground is thus connected a second resonant element Lr2/Cr2, and between the point A3 and the ground is thus connected a third resonant element Lr3/Cr3. Symmetrically between the point A4 and the ground is connected a resonant element Lr2/Cr2, and between the point A5 and the ground is thus connected a resonant element Lr1/Cr1. These “L/C series” elements create transmission zeros in the immediate neighbourhood of the cut-off frequency to increase the selectivity of the filter.
However according to a typical embodiment, to produce a filter discreet L/C components are used that are transferred onto a low-cost substrate, of FR4 type, the lower side of this substrate serving as a ground plane, and the grounding of these components being made by means of metallised holes.
It is known to those skilled in the art that the grounding of RF elements is never perfect and that this imperfection can be modelled to the first order by a parasite inductance. Concerning the grounding via metallised holes, the value of this inductance will depend notably on the diameter of holes and their depth. These ground parasite inductances can noticeably degrade the performances of a function, in which case it is necessary to take account and to compensate for them by re-optimizing the design. Thus, in the case of the filtering network of FIG. 1, the minimisation of ground parasite inductances is obtained by directly connecting the L/C series resonant elements to the lower ground plane of the circuit via metallised holes.
The HR-Si technology is today widely used for the integration of passive functions such as self-inductances, the capacities and resistances enable complete functions to be designed, such as for example filters, baluns, mixers and impedance transformers, with noticeable performances at the level of cost. It also enables the integration of systems on a technology known as SIP (System-In-Package), in which case the HR-Si technology serves not only to integrate RLC elements but also serves as support and interface to diverse integrated circuits constituting the system.
These performances are made possible due to the use of a high resistance silicon (HR-Si) typically of 1000Ω.cm. A structure of this type comprises a first layer of substrate HR-Si, typically of a thickness of 300 μm and a permittivity of the order of 11.7, metallised on one side, corresponding to the lower side. Above the other side corresponding to the upper side, two layers of metallic conductors are superimposed. Between the first metallic layer and the second upper layer is located an insulating layer, for example based on SiO2. This structure enables MIM (Metal-Isolation-Metal) capacities and coil inductances on the second upper layer to be implemented. The two layers are connected due to metallised crossings. On the upper side of the second metallic layer is located a passivation layer, for example, a layer of organic polymer BCB (benzocyclobutene).
However current technology does not enable metallised crossings to be produced between the first metallic layer and the lower side of the HR silicon constituting a ground plane. It thus does not enable the LC elements to be connected directly, that is to say by the shortest route, to the ground.
This limitation poses design problems in the particular case of complex functions, for example a high order filter comprising multiple elements to be grounded, as the parasite elements are such that they can completely degrade and denature the ideal response to the filter without there being a means of overcoming it.
FIG. 2 shows a cross-section of an embodiment of a passive filter in high resistance silicon in HR-Si technology, according to the prior art as described above and FIG. 3 shows the electric schema of such a low-pass filter. This standard filter comprises between the input and output ports P1 and P2, L/C series components represented by the capacities Cr1, Cr2, Cr3 and inductances Lr1, Lr2, Lr3 and inserted in parallel between the coupling inductors of values Ls1, Ls2, Ls3 and parasite ground lines M1, M2. The parasite ground lines M1, M2 between the ground points Gr1, Gr2 and Gr3, Gr4 respectively were modelled by the inductances Lm1, Lm2, Lm3, Lm4.
The response in transmission from this filter represented in FIG. 4 thus shows not only a shift of the cut-off frequency but also of transmission zeros. The attenuation beyond the cut-off frequency is on the other hand a lot lower comparatively with that of the ideal filter. The set of these degradations are due to the presence of these ground parasite inductances.
In the case of a measurement test points are used. These points are posed on the circuit as shown in FIG. 2, the central core on the input or output line, and the associated ground points at the ends of the ground lines.
The resonant elements Lr1/Cr1, Lr2/Cr2 and in particular Lr3/Cr3 can thus no longer be adjusted to the shortest ground points. These parasite ground lines Lm1, Lm2, Lm3, Lm4, of non-negligible length, as shown in FIG. 2 thus heavily modify the response of the filter, and are difficult to compensate for.
The present invention thus addresses the problem of grounding of constituent elements of this function when designed in HR-Si technology or any other technology that doe not enable vertical ground crosses to be produced.
Different solutions known in the prior art are proposed to resolve this problem.
A typical solution is to transfer this circuit in HR-Si technology onto the ground plane of a transfer substrate, and to connect in several points the parasite ground lines to this plane. This solution is illustrated by the circuit represented in FIG. 5 that show numerous wires, known as ground wires, connecting the ground lines to the ground plane enabling the effects of ground parasite inductances to be limited.
The circuit in HR-Si technology can also be mounted in flip-chip mode, this circuit being then returned on to the transfer circuit and connected to this latter using balls. Numerous balls are also necessary here to ensure a correct grounding of the filter.
Another solution known to the prior art is to design the filters in differential mode. This solution consists in transforming a filter referenced to the ground into a circuit in differential mode. The differential filter results in this case from a duplication of the non-differential filter according to a symmetry axis constituted by the global ground line. An example is provided in the thesis by M. L. GRIMA “Conception d'un récepteur radiofréquence en technologie intégrée pour les systémes de radioastronomie du futur” (Design of a radio-frequency receiver in integrated technology for future radio-astronomy systems), December 2007, Université d'Orléans.
It is clear that this solution presents a major disadvantage that is the doubling of components and from that of the silicon surface. Moreover it requires the use of transformers to convert the signals from differential mode into non-differential mode.